The present invention relates to a magnetomeasuring apparatus using superconducting quantum interference devices (SQUIDs) as magnetic field sensors, a method for assembling the same, a method for maintaining the same, and a diagnostic apparatus using the same.
Magnetomeasuring apparatuses using SQUIDs as magnetic field sensors are known. The SQUIDs have high sensitivities permitting the magnetomeasuring apparatus using the SQUIDs to measure weak biomagnetic fields.
One of such known magnetomeasuring apparatuses is a weak magnetic field measuring apparatus for measuring magnetic fields generated in, e.g., a head of a human, and is disclosed in Japanese Patent Laid-Open Publication No. Tokkai Hei 04-315075/1992. As shown in FIG. 16, a weak magnetic field measuring device 1 comprises a dewar vessel 2 including an inner vessel 2a and an outer vessel 2b. A bottom of the dewar vessel 2 is shaped in conformity with a head of a human being. A support shell 3 is accommodated in the bottom of the dewar vessel 2. The support shell 3 is formed in a shape contoured to the bottom of the dewar vessel 2. A plurality of SQUIDs 4 are arranged in the support shell 3, enclosing the bottom. The support shell 3 constitutes one unit 6 together with a mother board 5 in the form of a printed circuit board, etc. The unit 6 is submerged in liquid helium, a coolant. A neck plug 9 is disposed above the unit 6 on a support 8 and is housed in an upper part of the interior of the dewar vessel 2. The neck plug 9 comprises a radiation shielding and cabling unit. The support 8 has electric components mounted thereon.
The respective component members in the dewar vessel 2 are provided in units, and the unit 6 of the support shell 3 with the neck plug 9 and the support 8 can be removed from the dewar vessel 2 independently.
In order that a magnetomeasuring apparatus measures, with precision, a magnetic field over a wide range, it is necessary to arrange a number of magnetic field sensors over a wide range. Accordingly, the SQUIDs 4 of the weak magnetic field measuring apparatus 1 are mounted on the support shell 3 formed in a helmet-shape which can cover a head over a wide area. The support shell 3 must have an at least 30 cm-opening diameter, and the opening of the dewar vessel 2, through which the unit 6 including the support shell 3 is passed, has a diameter as large as above 30 cm.
Liquid helium, a coolant, is very volatile, and it is necessary to minimize intrusion of heat from the outside. Major routes of the heat intrusion into the dewar vessel 2 are the vacuum heat-insulating layer between the inner vessel 2a and the outer vessel 2b, and the opening of the dewar vessel 2. Loss of the liquid helium due to heat conduction in the opening and the wall of the dewar vessel 2a occupies a considerably high ratio, and the intruding heat reaches a considerably large total amount.
Thus, a large diameter of the opening of the dewar vessel 2 increases loss of the liquid helium in the dewar vessel 2 due to the intruded heat. Expensive liquid helium must be frequently supplied, which results in a disadvantage of higher maintenance costs.
An increase of a length of the opening of the dewar vessel 2 reduces heat conduction, but the dewar vessel 2 becomes taller. The taller dewar vessel 2 requires large rooms for installing the weak magnetic field measuring device 1.